The ventrobasal complex (VB) in the rat is a thalamic nucleus which processes several modalities of somatosensory sensation. Anatomically, it has a particularly simple structure, consisting almost entirely of thalamocortical relay cells (TCRs). Any differences in the processing of different modalities should show up as structural differences in the TCR cells. My aim is to determine whether TCR cells of different physiological properties have corresponding anatomical differences. This will be carried out by: 1) measuring the somal area, the extent, branching patterns and orientation of the dendritic tree, and the extent of any axon collaterals, and 2) by determining the relative distributions of different types of synapses along TCR dendrites. Individual cells will be indentified physiologically by intracellular recording, and then labelled by injection of horseradish peroxidase. Additional cells will be obtained by extracellular HRP injection or Golgi impregnations. Three-dimensional measurements of the labelled cells will be made on a computer microscope, allowing calculation of the relevant structural parameters. Some cells will be further studied ultrastructurally to determine synapse distributions. These studies should yield insights into the way functional properties of cells are reflected in their anatomical structure, a question of importance throughout the nervous system. They should also help determine what kind of processing goes on in this simple thalamic "relay nucleus." This information can serve as a basis for understanding the more complex circuitry found in the VB of higher mammals, including man.